This invention relates to a barrier layer and, more particularly, to a barrier layer to prevent diffusion of alkali metal ions, such as sodium ions, from a glass substrate into a medium e.g. a coating such as an electroconductive coating or a photocatalytic coating.
Alkali metal ions, e.g. sodium ions in glass, particularly at elevated temperatures, migrate from the surface of the glass into the medium overlaying the glass. For example, in liquid crystal display (xe2x80x9cLCDxe2x80x9d) devices similar to the type disclosed in U.S. Pat. No. 5,165,972, the sodium ions in the surface of the glass substrate migrate into the liquid crystal material causing deterioration of the liquid crystal material. Further, in electrochromic displays, the sodium ions migrate into coatings overlaying the surface of the glass substrate and/or into the electrolyte which causes deterioration of the coating and/or electrolyte. During fabrication of LCD devices and/or electrochromic devices, the glass substrate is heated to temperatures as high as 1100xc2x0 F. (593xc2x0 C.) to seal the devices; during such heating sodium ion migration is accelerate.
Unless hindered, the sodium ions migrate into the medium e.g. the electroconductive coating, the electrolyte and/or the liquid crystal material overlaying the surface of the glass substrate deteriorating the medium.
It is also believed that alkali metal ion migration, e.g. sodium ion migration, also causes deterioration of 5 photocatalytic compositions of the type disclosed in International Application Publication No. WO 95/11751, in photocatalytically self cleaning coatings of the type disclosed in U.S. patent application Ser. No. 08/899,257 filed on Jul. 23, 1997, in the names of Charles B. Greenberg et al. for xe2x80x9cPHOTOCATALYTICALLY-ACTIVATED SELF-CLEANING ARTICLE AND METHOD OF MAKING SAMExe2x80x9d and in photoelectrolytically reducing coating of the type disclosed in U.S. patent application Ser. No. 08/927,130 filed on Sep. 2, 1997, in the name of James P. Thiel for xe2x80x9cPHOTOELECTROLYTICALLY-DESICCATING MULTIPLE-GLAZED WINDOW UNITSxe2x80x9d. In general, compositions include titanium dioxide or zinc oxide particles held together and to a glass substrate by a silicone binder or coatings of titanium oxides, iron oxides, silver oxides, copper oxides, tungsten oxides to name a few. The surface of the composition and film can act as a biocidal agent under the application of light.
One technique to prevent or minimize alkali metal ion migration is to provide a barrier coating between the medium and the glass substrate.
U.S. Pat. No. 5,165,972 to Porter discloses barrier coatings to prevent migration of alkali metal ions from a glass surface. The barrier coating is deposited by pyrolysis of a silane gas on the glass surface above 600xc2x0 C. in the presence of a gaseous electron donating compound. Oxygen from the glass is incorporated with silicon to form a transparent barrier coating up to 50 nanometers thick on the glass surface to prevent migration of alkali metal ions into overlying layers sensitive to alkali metal ions. Although the technique of Porter ""972 is acceptable, there are drawbacks. For example, oxygenation by pyrolysis requires high energy inputs especially if the sheets have to be heated before coating making the process expensive.
U.S. Pat. No. 4,238,276 to Kinugawa discloses a barrier layer that includes silicon oxide, titanium oxide, aluminum oxide, zirconium oxide, magnesium oxide and nickel oxide. Kinugawa discloses silicon oxide barrier coating having a thickness of 1000 angstroms. Although the barrier coating disclosed by Kinugawa is acceptable, it has drawbacks. More particularly, depositing a 1000 Angstrom thick layer of silicon oxide by any technique is more expensive than depositing a silicon oxide layer less than 1000 Angstroms thick by the same process. Further, a thin silicon oxide layer of the type disclosed in Kinugawa may not act as an effective barrier.
European Patent Specification Publication No. 0 071 865 B to Mizuhashi et al. discloses a glass body having an alkali-containing glass substrate and a silicon oxide layer formed on its surface for preventing diffusion of alkali metal ions from the glass substrate. The silicon oxide layer has from 0.01 to 25 molar percent hydrogen bonded to silicon. Although the technique disclosed by Mizuhashi et al. appears to prevent alkali metal ion migration, there are drawbacks. More particularly, the barrier coating may trap hydrogen gas which may escape during fabrication/use of the product e.g. LCD devices. As can be appreciated, it is preferred not to have a coating that may randomly release hydrogen gas into a medium which may result in deterioration of the medium. Further, hydrogen that is chemically bonded in the coating may affect optical and mechanical properties of the coating.
As can be appreciated, it would be advantageous to provide a thin barrier layer that can be applied economically, and does not have the drawbacks/limitations of the presently available technology.
The present invention recognizes the desirability of utilizing a thin material as a diffusion barrier for alkali metal ions such as sodium ions. Although the prior art suggests that the refractive index of such a diffusion barrier should match the refractive index of the substrate as closely as possible, thus selecting silica for glass substrates, in accordance with the present invention, very thin layers of metal oxides such as zirconium oxide, titanium oxide and zinc/tin oxide are produced as effective diffusion barriers for sodium ions without compromising optical properties of the coated glass.
In general the present invention relates to an article having a medium e.g. photocatalytic coating, water reducing coating, electroconductive coating, electrolyte of a photochromic device and/or liquid material of liquid crystal display over and spaced from the surface of a glass substrate. A barrier layer e.g. zirconium oxide, titanium oxide or zinc/tin oxide is deposited by magnetron sputtering over the glass substrate to provide a barrier layer between the medium and the glass substrate. The barrier layer or film is a thin amorphous film and has a density equal to or greater than about 75% of the crystalline density of the metal oxide of the film in the practice of the invention and the barrier films are in the range of 30 to 180 Angstroms depending on the barrier film selected. While zirconium oxide, titanium oxide and zinc/tin oxide have refractive indices significantly higher than the refractive index of typical glass substrates, because they are very thin there is no deleterious effect on the optical properties of the coated glass substrate.
The glass substrate having the barrier layer may be used as a component of a liquid crystal display cell, and/or of a photochromic device and/or have a photocatalytic film deposited thereover.